We propose to study lens coupling between the phosphor and the CCD sensor in X-ray detectors for macromolecular crystallography. In this project we will develop three technologies: large aperture lenses; high gain phosphors; and back-illuminated, large-format CCDs. We have developed a prototype big lens, and we are now characterizing it. We have developed a new phosphor (ZnSe:Cu,Ce,CI) with significantly higher gain and less afterglow than conventional Gd2O2S:Tb. We propose to study and improve on this technology, specifically to find a phosphor that is even faster and brighter, and does not contain selenium. We have begun a program to thin a 61mm CCD for back illumination, thus increasing light conversion efficiency in these large CCDs. We propose to develop ever larger CCDs with faster readout and lower noise, that are back-illuminated. Lens-coupling optics eliminates zingers, dead spaces, and the inherent defects of fiberoptic coupling optics (chickenwire, shear distortion, nonuniform optical transmission). Lens coupling reduces spatial distortions and positional nonuniformities relative to fiberoptic coupling. The point response should be better. With improvements in CCD efficiency and phosphor gain that we expect to realize, and the optical transfer efficiency that these very large lens systems can achieve, the overall system gain (electrons stored in the CCD/incident X ray) should be comparable to or superior to fiberoptic coupling. The detective quantum efficiency (DQE) and dynamic range should also be equivalent or superior to modular mosaic CCD detectors. In production, this technology should be substantially cheaper than mosaic CCDs.